New, energy-saving, efficient and cost-effective processing technologies such as 2D and 3D inkjet printing (IJP) for the production and integration of intelligent components will be opening up very interesting possibilities for industrial applications of molecular materials in the near future. Beyond the use of home and office based printers, "inkjet printing technology" allows for the additive structured deposition of photonic and electronic materials on a wide variety of substrates such as textiles, plastics, wood, stone, tiles or cardboard. Great interest also exists in applying IJP in industrial manufacturing such as the manufacturing of PCBs, of solar cells, printed organic electronics and medical products. In all these cases inkjet printing is a flexible (digital), additive, selective and cost-efficient material deposition method. Due to these advantages, there is the prospect that currently used standard patterning processes can be replaced through this innovative material deposition technique. A main issue in this research area is the formulation of novel functional inks or the adaptation of commercially available inks for specific industrial applications and/or processes.
In this contribution we report on the design, realization and characterization of novel active and passive inkjet printed electronic devices including circuitry and sensors based on metal nanoparticle ink formulations and the heterogeneous integration into 2/3D printed demonstrators. The main emphasis of this paper will be on how to convert scientific inkjet knowledge into industrially relevant processes and applications.
Due to the increasing demand for storage capacity in various electronic gadgets like mobile phones or tablets, new types of non-volatile memory devices have gained a lot of attention over the last few years. Especially multilevel conductance switching elements based on organic semiconductors are of great interest due to their relatively simple device architecture and their small feature size.
Since organic semiconductors combine the electronic properties of inorganic materials with the mechanical characteristics of polymers, this class of materials is suitable for solution based large area device preparation techniques. Consequently, inkjet based deposition techniques are highly capable of facing preparation related challenges. By gradually replacing the evaporated electrodes with inkjet printed silver, the preparation related influence onto device performance parameters such as the ON/OFF ratio was investigated with IV measurements and high resolution transmission electron microscopy. Due to the electrode surface roughness the solvent load during the printing of the top electrode as well as organic layer inhomogeneity’s the utilization in array applications is hampered. As a prototypical example a 1diode-1resistor element and a 2×2 subarray from 5×5 array matrix were fully characterized demonstrating the versatility of inkjet printing for device preparation.
For the emerging fields of biomedical diagnostics and environmental monitoring, where sensor platforms for in-situ sensing of ions and biological substances in appropriate aqueous media are required, electrolyte-gated organic fieldeffect transistors (EGOFETs) seem to be the transducers of choice. Due to the formation of an electric double layer at the electrolyte/organic semiconductor interface, they exhibit a very high capacitance allowing for low-voltage and waterstable operation. In combination with the outstanding properties of organic devices like biocompatibility, lowtemperature processability on flexible substrates, as well as the possibility to tune the physical and chemical properties enhancing the selectivity and sensitivity, EGOFET-based sensors are a highly promising novel sensor technology. In order to obtain a reliable sensor response, a stable device operation is crucial. Within this context, we present a combined study of poly(3-hexylthiophene)–based EGOFETs on various substrates. In particular, the influences of different concentrations of NaCl in the electrolyte and various gate electrode materials, to tune the threshold voltage have been investigated. Furthermore, the limits of the stable operational window are evaluated and the effects when abandoning the latter are discussed.
We report on the influence of photolithographic processing of source/drain electrodes on the device performance of
regioregular poly(3-hexylthiophene)-based bottom-gate bottom-contact organic field-effect transistors (OFETs). The
presented results demonstrate that it is not only the processing conditions of the organic semiconductor influencing
relevant device parameters, but it is also the preceding process steps including the structuring of electrodes via lift-off
which significantly determine the OFET performance. In particular, the effects of photoresist residuals within the active
channel region and the influence of the application of various lift-off chemicals were thoroughly investigated by contact angle measurements, atomic force microscopy and electrical characterization of OFET-based devices. By modifying the dielectric/semiconductor and/or electrode/semiconductor interfaces, the applied chemicals are shown to affect the device performance in terms of switch-on voltage, subthreshold swing and on/off-current ratio. The present study emphasizes the necessity for the optimization of the manufacturing process in order to obtain reproducible high-performing OFETs and OFET-based sensors.
For high-performance low-cost applications based on organic field-effect transistors (OFETs) and corresponding sensors
essential properties of the applied semiconducting materials include solution-processability, high field-effect mobility,
compatibility with adjacent layers and stability with respect to ambient conditions. In this combined study regioregular
poly(3-hexylthiophene)- and pentacene-based bottom-gate bottom-contact OFETs with various channel lengths are
thoroughly investigated with respect to short-channel effects and the implications of dielectric surface modification with
hexamethyldisilazane (HMDS) on device performance. In addition, the influences of oxygen, moisture and HMDStreatment
on the ambient stability of the devices are evaluated in detail. While OFETs without surface modification
exhibited the expected degradation behavior upon air exposure mainly due to oxygen/moisture-induced doping or
charge-carrier trapping, the stability of the investigated semiconductors was found to be distinctly increased when the
substrate surface was hydrophobized. The presented results thoroughly summarize important issues which have to be
considered when selecting semiconducting materials for high-performance OFETs and OFET-based sensors.
Aside from other target applications, organic field-effect transistors (OFETs) are also promising devices for sensing
various kinds of analytes, including gases, ions and biomolecules. In this work ion-sensitive polymer-based OFETs will
be discussed. In detail, operational device instabilities caused by the movement of mobile ions in
poly(3-hexylthiophene)-based OFETs are investigated, when (a) an
ion-containing gate dielectric, polyvinyl alcohol (PVA), is
applied in a top-gate architecture and (b) ions are deliberately added to the organic semiconductor in a bottom-gate
architecture. The underlying mechanisms for the observed
source-to-drain channel current drifts upon bias stress are
thoroughly explained. In addition, device instabilities due to mobile ions within the dielectric are demonstrated with rigid
and flexible PVA-based OFETs including inkjet-printed source/drain electrodes and a meander-shaped top-gate
architecture, the latter enabling the realization of smart, integrated and low-cost OFET-based sensor systems.
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